1. Field of the Invention
The invention relates to an apparatus and method for determining the speed and position of an object. In particular, the invention relates to a device incorporating a rangefinder, inclinometer, compass and global positioning system receiver, wherein the device is capable of determining the location and/or speed of a target object.
2. Background of the Related Art
A laser rangefinder allows an operator to determine the distance to a target object with great accuracy. A laser rangefinder that incorporates a compass and an inclinometer can also enable an operator to determine the bearing (azimuth) and height (elevation), respectively, of a target object with great accuracy.
Such laser rangefinders have found many applications in surveying and mapping. In surveying, a single surveyor, instead of two, can measure distance, bearing, and inclination with great accuracy, as illustrated in U.S. Pat. No. 5,291,262 to Dunne, the contents of which are hereby incorporated by reference.
Laser rangefinders have also been incorporated in speed determination devices, such as the laser speed detectors used by law enforcement personnel. Examples of such devices are disclosed in U.S. Pat. No. 5,221,956 to Patterson et al. and U.S. Pat. No. 5,359,404 to Dunne, the contents of both of which are hereby incorporated by reference. In these devices, the rangefinder determines the distance to a target object at a plurality of different points in time. The determined distances and the elapsed times between measurements are then used to calculate the speed of the target.
In laser rangefinding and speed determination, typically, a short duration infrared laser light pulse is transmitted from the laser rangefinder to the target. The target reflects a portion of the laser pulse back to the laser rangefinder. The laser pulse transmitted from the rangefinder will diverge as it travels from the laser rangefinder to the target. After the laser pulse is reflected from the target, it will further diverge as it travels back toward the rangefinder. The power of the reflected laser pulse that is detected by the laser rangefinder is therefore a function of the effective solid angle subtended by the detecting portion of the laser rangefinder relative to the target, the divergence of the beam, the initial beam intensity and the reflectivity of the target.
Typically, a time period is measured between the time a transmitted pulse leaves the laser rangefinder and the time the reflected pulse from the target is received back at the laser rangefinder. This time period, which is typically called the time of flight, allows the rangefinder to determine the distance between the rangefinder and the target. To determine when the time period starts, some rangefinders cause a portion of the transmitted laser pulse to be redirected to a detector. The detector can then generate a signal that represents the firing time (zero time reference) of the transmitted pulse, as illustrated in the above-cited '262 patent. When a reflected light pulse is received back at the rangefinder, the detector will generate another signal that represents the end of the measured time period.
Most rangefinders use a counter to determine the time period that elapses between transmission of a pulse and receipt of the reflected pulse. The counter will count the number of clock signals that are output from a clock device between the time the transmitted pulse leaves the rangefinder, and the time the reflected laser pulse is received back at the rangefinder. Unfortunately, simply counting clock pulses and converting the number of pulses into a time period will not provide the degree of time keeping accuracy required to provide precise position or range measurements.
Because a fractional time period will usually elapse between the point in time when a laser pulse leaves the rangefinder and the point in time when the clock outputs a first signal, failing to account for the fractional time period can lead to inaccuracies in measurement. Likewise, inaccuracies can result if the rangefinder fails to account for a fractional time period that elapses between the point in time when the clock outputs its last signal and the point in time when a reflected laser pulse is received back at the rangefinder. To ensure the most accurate distance measurement, it is necessary for the rangefinder to have some means of determining the length of the fractional time periods.
Various methods for viewing a target and for displaying relevant range, azimuth, inclination, and speed information have also been disclosed. This includes using a magnifying scope for viewing the target, and using a separate display for displaying target information. Because the target and the target information are displayed at different locations, an operator viewing the target must reposition his head and refocus his eyes, or at least refocus his eyes, to view target information, or vice versa. In either case, this approach requires an operator to take his eyes off the target while viewing the relevant target information. Thus, the operator is not permitted to simultaneously view the target and the target information. Lack of simultaneity is a disadvantage because possible misalignment of the laser rangefinder may occur when the target is not in the operator's view.
In contrast, a laser rangefinder incorporating a head-up display (HUD) allows a user to simultaneously view the target and target information. Typically head-up displays have been designed to permit an operator to view two different inputs of information at the same time. HUDs are time saving and have been successfully employed in many military applications.
The above-cited '956 patent discloses a rangefinder that incorporates a HUD specifically designed to work with an off-the-shelf light emitting diode (LED). The head-up display projects target information to an operator while the operator simultaneously views a target. However, in the '956 patent, a two lens element folded telephoto system having a folding mirror in the optical path is used to project the target information from the off-the-shelf LED readout to the operator by way of the HUD. The telephoto system disclosed in the '956 patent provides an effective focal length much greater than the physical length of the lens system to save space, while forming an image of the off-the-shelf LED.
Although a folded telephoto lens system for a head-up display can save space by providing a reduced physical length, it has several important disadvantages. First, such a system requires two separate focusing lenses: a positive power doublet lens, and a negative power lens. The system also requires a folding mirror as an additional optical element. The use of two focusing lenses and a folding mirror adds bulk, weight and cost to the overall system. Moreover, because this system is a compound lens system, the optical elements must be carefully aligned with respect to one another. A compound lens system may also be more fragile, and certainly has more elements to break or misalign. For example, a compound lens system may require exceptionally careful optical re-alignment if dropped to the ground or if accidentally struck, compared to lens systems avoiding compound lenses or folding mirrors. The above-cited disadvantages and problems are particularly problematic in a device intended for field use at construction and blast sites.
Typically, an image of a target object must be transmitted through a combining element of a head-up display, while an image of target information is projected onto and is reflected from the combining element. Head-up displays can have problems optimizing the transmissivity and reflectivity of the combining element that presents the user with images of both the target object and relevant target data. Previous laser rangefinders employing HUDs, including the one disclosed in the '956 patent, have not optimized these properties for both the prominent visible (photopic) wavelengths reflected from a target and the prominent visible wavelengths projected from a target information display.
In addition, some HUDs suffer from chromatic shading or shifting of a target image when viewed through the HUD. Typically, the target image is shifted toward the blue end of the spectrum. The target image appears bluish and darkened to an operator because the required filtering removes red light from the light that passes through a viewfinder to the operator's eye. This darkening of the target image is obviously disadvantageous for good viewing of the target image by the operator.
The devices disclosed in the '956 and '262 patents use a backlit targeting reticle (or "cross-hairs"). This is disadvantageous because it requires a light source in addition to the respective reticles themselves. In addition, the device disclosed in the '956 patent uses off-the-shelf LEDs, separate from the device that creates an image of the reticule, to create an image of target information. The LEDs are necessarily mounted on a different substrate than the reticle, which increases the cost, weight and size of the device. Moreover, these LEDs, being off-the-shelf, are not specifically matched to the folded telephoto lens system employed therein. Such a system is complex, and does not provide optimum optical clarity.
There are other important issues that require attention in laser rangefinders. One problem is the effect temperature variations have on optical and mechanical elements. The expansion and contraction that occurs due to temperature variations can result in significant imprecision in measurements of relevant target information, or loss of range. In addition, previous laser rangefinders have required the use of a separate power pack containing a power supply or batteries. This also tends to add cost, weight and complexity to the system.
Prior art laser rangefinders have utilized the functions of the Global Position System (GPS) to locate the exact position of measured objects. Typically, a separate GPS receiver is connected to a rangefinder via a data cable through a data input port. The rangefinder uses the location of the GPS receiver, and information derived from its own sensors, to determine the position of a measured object. Unfortunately, this arrangmenet requires the use of two separate devices, the rangefinder and a separate GPS receiver, which is cumbersome and impractical for some types of field work.
Rangefinders used in survey work have also been used to download data to a separate storage device, such as a computer or a portable data storage device. Typically, the rangefinder includes a data output port which is connected to the separate storage device via a data cable. The storage device runs a program which captures and stores information generated by the rangefinder. In the past, no rangefinder has included a simple or convenient integral means for automatically recording data generated while performing rangefinding, surveying or speed determining functions. It has always been necessary to download such data to a separate storage device through a data cable, which is cumbersome and requires extra equipment.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.